add(rpc): Adds `getblockheader` RPC method (#8967) #2186

Job	Run time
Test stable on ubuntu-latest	31m 24s
Test beta on ubuntu-latest	32m 9s
Test stable on macos-latest	31m 31s
Test stable on windows-latest	1h 2m 21s
Test beta on windows-latest	1h 0m 58s
Check Cargo.lock is up to date	56s
Install zebrad from lockfile without cache on ubuntu-latest	8m 32s
Check deny.toml bans	44s
Check for unused dependencies	7s
Check deny.toml bans --features default-release-binaries	44s
Check deny.toml bans --all-features	53s
Check deny.toml sources	48s
Check deny.toml sources --features default-release-binaries	48s
Check deny.toml sources --all-features	49s
Open or update issues for OS integration failures	0s
	3h 52m 44s

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